SPOILERS
At the end, what did the salesman see that was the "sign" that the old man had told him about? I love Wolfe's subtlety, but occasionally I just totally don't get it.
Also, am I alone in interpreting that last scene as the other three having already used the device and become their "improved" selves, whereas the salesman had waited to get his son first and was about to "merge" with himself by stepping through the door?
Michael Straight